This invention relates generally to the production of acrylic acid by vapor-phase catalytic oxidation of propylene in two stages. More particularly, the invention relates to an industrially safe and economical process for producing acrylic acid by the above mentioned oxidation in which a number of hitherto unsolved problems, as described hereinafter, are overcome by the use of a specific catalyst and the selection of specific reaction conditions or operational conditions in the first-stage reaction and by the supply of oxygen by a specific mode and the selection of specific reaction or operational conditions in the second-stage reaction.
As a process for producing acrylic acid, the method of vapor-phase catalytic oxidation of propylene in two stages with the use of air is known and is already being industrially practiced. In the first stage of this process, propylene is mixed with air and steam or an inactive gas such as nitrogen and then the mixed gas is supplied thereby to convert the propylene principally into acrolein and a by-product quantity of acrylic acid, and the outlet gas of this first stage is supplied, without separation of the formed products, to the reaction vessel of the second stage, in which the acrolein is principally converted into acrylic acid. The acrylic acid thus formed is, in general, cooled and recovered as an aqueous solution from the gas stream and, in the succeeding purification step, is subjected to an extraction, distillation, or like process step and is thereby isolated. As an alternative procedure, a method wherein the gas stream from the outlet of the second stage is cooled, and then acrylic acid is absorbed with a suitable solvent and thereby separated has been proposed.
In this series of process steps, the performance of the oxidation catalyst thereof greatly influences the production economy, and, accordingly, numerous proposals with respect to the catalysts of each stage have been made. It may be considered that the performances of these catalysts in terms of the yield of the objective acrolein or acrylic acid are generally amply high on the economical view point. The production of the objective products with high yields of 90 percent or higher are reported, for example, in Japanese Patent Publication Nos. 17711/1972, 27490/1972, 41329/1972, 42241/1972, 42813/1972, 1645/1973, 4763/1973, 4764/1973, and 4765/1973 with regard to first-stage catalysts and, with regard to second-stage catalysts in Japanese Patent Publication Nos. 12129/1969, 19296/1973, 169/1974, 11371/1974, 10432/1977, and 31326/1977 and in Japanese Patent Laid Open Nos. 2011/1971, 8360/1972, 43922/1974, 61117/1974, 124016/1974, 133317/1974, 25520/1975, 93918/1975, 23589/1977, 29483/1977, and 29484/1977. However, in the case of industrial production of acrylic acid with the use of these catalysts, various difficulties are encountered with the realization of necessary industrial conditions other than the yield of the objective product.
One of these problems is that, although there is the necessity of obtaining the objective product with an amply high productivity, that is, space time yield, if the propylene concentration in the starting material is made high for this purpose, there will be a restriction due to the explosion limit, and a quantity of oxygen sufficient for obtaining the objective product cannot be supplied. Consequently, the conversion of the propylene or acrolein will drop, and the single-pass yield will decrease, or hot spots are readily produced at inlet part of the catalyst bed, whereby there is the danger of a runaway temperature rise, and, in addition, a deterioration of the catalyst is caused by the excessive generation of heat.
In this connection, as a measure for preventing the production of hot spots, the measure, simply stated, of diluting the catalyst at parts of great generation of heat with another inactive material, as described in Japanese Patent Publication No. 9859/1959, for example, has been known as a general method. A measure for this purpose with the object of producing acrylic acid is disclosed in, for example, Japanese Patent Laid Open No. 127013/1976.
On the other hand, propylene and acrolein upon being mixed with air produce explosive gases. For this reason, in order to avoid the forming of these explosive gases and, at the same time, to introduce oxygen in the quantity necessary for the reaction, it has been the general practice to add an inactive gas such as nitrogen or steam, particularly steam. However, when the quantity of steam is great, the concentration of the aqueous solution of acrylic acid obtained decreases, and, as a consequence, disadvantages such as an increase in the cost of separating the acrylic acid or an increase in the recovery loss arise.
As a solution for these problems, the method of recycling the residue gas remaining after the acrylic acid and water have been recovered and separated by cooling, absorption or scrubbing with a solvent or like measure from the gases formed in the reaction (the residue gas comprising, principally, nitrogen, carbon dioxide, carbon monoxide, etc., and, in addition, unreacted propylene, acrolein, oxygen, etc.), substituting this residue gas for steam thereby increasing the acrylic acid concentration of the aqueous solution thus obtained naturally appears to be suitable and has been disclosed in, for example, Japanese Patent Publication No. 30688/1978 and Japanese Patent Laid Open Nos. 36415/1976, 108917/1977, 15314/1978. However, since the composition of the exhaust gases depends on the reaction conditions and thus fluctuates, this method is accompanied by the problem of complicated operational control and procedure for avoiding the explosive range due to this gas recycling and of the danger.
As a measure for increasing the space time yield, the method of carrying out the reaction by increasing the space velocity thereby to shorten the contact time appears at first sight to be suitable. However, since the reaction temperature in this case is high as a natural result, the selectivity of the reaction tends to decrease, and, in addition, the catalyst life is shortened. Furthermore, operation on the high-temperature side is disadvantage also because of combustion reaction of runaway character in the empty parts of the reaction vessel due to spontaneous or autogenous oxidation of the acrolein described hereinafter.
Another method of increasing the space time yield is to use a high reaction pressure thereby to increase the process quantity per unit time. This method is effective up to a certain pressure range. However, when the pressure becomes high, the molecular diffusion velocity decreases. For this reason, the diffusion resistance between the bulk of the gas stream and the catalyst surface increases, and, as a result, the yield of the objective product is lowered, whereby the use of this method is limited.
A second difficult problem accompanying the production on an industrial scale of acrylic acid by catalytic oxidation of propylene in two stages is how to avoid or lessen the danger of occurrence of a runaway combustion reaction, which may be considered to arise from spontaneous oxidation of acrolein, at the outlet vapor-phase part of the first-stage reaction. As a result of our experiments, we have found that this combustion occurs when acrolein coexists with oxygen and that the higher the gas temperature, the higher the partial pressure of the acrolein, and the longer the retention time because of large space volume, the higher will the reaction rate become and give rise to a rapid and violent combustion.
Accordingly, in order to attain a high space time yield and carry out production under safe conditions, a correspondingly proportionate countermeasure technique is required. As measures intended to solve these problems, the following methods, for example, are known. Japanese Patent Laid Open No. 36415/1976 discloses a method which comprises cooling the outlet gas stream of the second stage in the method of oxidizing propylene in two stages, recovering the acrylic acid formed in an aqueous solution thereof, thereby separating and recovering the same, and recycling the residual gas stream by dividing it into portions respectively for the first and second stages. As a countermeasure for the acrolein combustion at the first-stage outlet in this method, it is proposed to "add and mix air and waste gas to and with the reaction gas mixture immediately after it has come out of the catalyst zone of the first stage with rapid cooling to a temperature of the mixture of 150.degree. to 320.degree. C.".
This disclosure suggests an effective method as one countermeasure for the acrolein combustion, but, as far as we are aware, since it is necessary to supply to the first and second stages the waste gas and air in quantities controlled to be within specific ranges, a very complicated control procedure is required when carrying out an unsteady-state operation such as that during start-up or shut-down of the plant while avoiding the explosive range, and the operation entails danger. Particularly since the quantity of residual oxygen in the waste gas differs with the reaction rate of each stage as mentioned hereinbefore, it is necessary to constantly monitor the outlet oxygen concentration and responsively control the air supply rate, the recycled gas flow rate, the reaction temperature, and other variables, whereby the operational procedure would be complicated.
Furthermore, the specified temperature range of 150.degree. to 320.degree. C. can include the range wherein combustion suppression is impossible and may thus be interpreted to be inadequate with respect to the object. While the method described in Japanese Patent Laid Open No. 15314/1978 may also be effective, it is not different, as far as we are aware, from the method described in Japanese Patent Laid Open No. 36415/1976 on the point that the formation of the explosive range in the first-stage reaction is avoided by recycling waste gas.
Thus, while the various proposals described above respectively indicate improvements with respect to problems, it is our belief that none of them can yet be said to be satisfactory from the industrial viewpoint.